In color photography the dye image is formed during a color developing process. The color developing agent, usually a N,N-dialkyl-p-phenylenediamine is oxidized in a reaction with silver halide producing free silver. Oxidized developing agent then reacts rapidly with a variety of organic molecules, called couplers, to form dyes. The color of the dye thus formed is determined by the generic structure of the coupler.
In the preparation of silver halide emulsions for recording colors, it is customary to incorporate a color forming material in the silver halide emulsion so that upon color development this color forming compound (coupler) reacts with the oxidation product of the developing agent to form a dye. In the typical format for recording the visible spectrum, the emulsion is coated in several layers, each sensitive to a particular region of the spectrum. By suitable control of the development and by use of several couplers, it is possible to form the proper dye in each layer to give an adequate color representation of the photographed scene or object.
The greatest sharpness in three simultaneously prepared color separations in permanently superimposed emulsions is obtained if the reduced silver, or the chemical components which are substituted in the place of the reduced silver or in the place of the residual silver halide or even better deposited during development -- are employed in the color separations. To this end these chemical compounds must be insoluble or at least not easily diffusing and they, or the silver, must have reducing, oxidizing or catalytic properties. Therefore numerous methods of color photography are possible by means of triple or double emulsion layers containing the color generators from the beginning.
It has already been proposed for the production of color photographs to add leuco-dyes to a silver salt emulsion and to convert these into a particular color as a result of the interaction between the silver reduced upon development and the leuco-dyes, whereby the locally deposited quantity of silver in the developed picture determines the degree of color formation for every point in the sensitized layer. Problems arise in the reduction of this proposal to practice due to the inherent oxidative instability of the leuco-dyes. Common leuco-dyes incorporated in sensitized layers of a film generally oxidize on keeping and cause indiscriminate dye formation.